An organic electroluminescence device capable of emission with a luminance of about 1000 cd/m2 at a driving voltage equal to or lower than 10 V was reported by Tang et al. of Eastman Kodak in 1987. See Appl. Phys. Lett., vol. 51, p. 913, 1987.
Tang et al. searched for the optimal electrode and organic compound that would increase the emission efficiency of the device. They prepared an organic electroluminescence device in which the hole transport layer is an aromatic amine compound and the electron transport emission layer is an aluminum complex of 8-hydoxyquinoline. This device emits light when it has DC voltage applied across its electrodes. The DC voltage injects holes through the anode and electrons through the cathode, so that they undergo recombination in the emission layer to emit light.
Among the known hole transport materials are aromatic diamine derivatives (disclosed in Japanese Patent Laid-open Nos. Hei 8-20771, Hei 8-40995, and Hei 8-40997) and aromatic amine-containing polymers (disclosed in Japanese Patent Laid-open Nos. Hei 11-283750 and 2000-36390).
Among the known hole injection materials are phthalocyanine derivatives and aromatic triamines of starburst type (disclosed in Japanese Patent Laid-open Nos. Sho 63-295695 and Hei 4-308688).
The existing organic electroluminescence devices have some problems to be addressed. That is, they need to have a longer life, to work at a lower driving voltage, and to work with a constant current without voltage increase and luminance decrease. Although no elucidation has been made yet, the performance of the device seems to depend on the properties (such as glass transition temperature and melting point) of the organic materials constituting the device. The device made of an organic material poor in heat resistance tends to break during continuous operation.
Any device with a short life or a high driving voltage poses a problem when used as the light source for facsimile, copying machine, and back light of liquid crystal display, particularly, the device is not desirable for display.
It is an object of the present invention to provide an organic electroluminescence device capable of high luminance emission at a low driving voltage (for a longer life) and a material used for the device.